This invention relates to microelectromechanical systems (MEMs), and more particularly relates to electrostatically-actuated structures for MEMs.
MEMs are increasingly being employed for a wide range of applications, in part due to the ability to batch fabricate such microscale systems with a variety of highly complex features and functions. Microscale sensing and actuation applications are particularly well-addressed by MEMs For many MEMs applications, electrostatically-actuated structures are particularly effective as analog positioning and tuning components within complex Microsystems. Electrostatic actuation provides a combination of advantages for the microscale size regime of MEMs, including the ability to produce high energy densities and large force generation, as well as the general ease of electrostatic actuator fabrication, and high operational speed due to relatively small mass. Indeed, for many MEMs applications, electrostatic actuation is preferred.
Electrostatic actuation of a structure is typically accomplished by applying a voltage between an electrode on the structure and an electrode separated from the structure. The resulting attractive electrostatic force between the electrodes enables actuation of the structure toward the separated electrode. This applied electrostatic force is opposed by a characteristic mechanical restoring force that is a function of the structure's geometric and materials properties. Control of the structure's position during actuation requires balancing the applied electrostatic force and inherent mechanical restoring force.
The electrostatic force is a nonlinear function of distance; as the structure moves toward the separated electrode, such that the electrodes' separation distance decreases, the electrostatic force between the electrodes typically increases superlinearly. In contrast, the mechanical restoring force of the structure typically is a linear function of distance. Due to this disparate dependence on distance, not all positions between the electrodes are stable. Specifically, at electrode separations less than some minimum stable separation characteristic of the structure, the structure position is unstable and causes uncontrollable travel of the structure through the remaining distance to the separated electrode. This instability condition, known generally as "pull-in," is a fundamental phenomenon resulting from the interaction of the nonlinear electrostatic force with the linear, elastic restoring force of the structure being actuated. Generally characteristic of a electrostatically-actuated structures, pull-in instability is well-known to severely limit the fraction of an electrode separation gap through which such a structure can be stably positioned.
A separate but related limitation of electrostatically-actuated structures is the relatively high voltage level typically required to position such a structure through a relatively large stable actuation range. As a result of this limitation, in combination with the electrostatic pull-in limitation, electrostatically-actuated structures typically are not well-suited to produce a large range of actuation motion. But for many microscale positioning and tuning systems, such as optical systems, large ranges of travel, and analog tuning of position, can both be critically required. There thus remains a need for electrostatic actuation techniques that enable large ranges of travel and further that are optimized for actuator operation at the lowest possible operational voltage.